To fix a malfunctioning keyless entry system on a 2017 Mercedes-Benz C-Class, you can follow these detailed steps:

Check the Key Fob Battery

• Start by replacing the battery in the key fob. Over time, batteries lose their charge and can cause the keyless entry system to malfunction.
• Carefully open the key fob using a small flathead screwdriver. Look for a notch or seam where the halves of the fob meet and gently pry them apart.
• Note the orientation of the old battery before removing it. This will help ensure you insert the new battery correctly.
• Insert a new battery of the same type, typically a CR2032, and reassemble the key fob.
• Test the key fob by pressing its buttons to see if the issue is resolved.

Inspect for Interference

• Identify any potential sources of radio frequency interference. Nearby radio towers, security systems, and electronic devices can affect the keyless entry system.
• Move away from areas with heavy interference and test the key fob again.

Check the Vehicle's Battery

• A weak or dead vehicle battery can also cause keyless entry malfunctions. Make sure the car's battery is properly charged and functioning.
• If the battery is old, consider having it tested and replaced if necessary.

Reset the Keyless Entry System

• Standing close to the vehicle, press the lock and unlock buttons on the key fob simultaneously for several seconds until the car's lights flash.
• Test the key fob after the reset to see if functionality has been restored.

Reprogram the Key Fob

• Consult the owner's manual for the specific reprogramming procedure for your model if the reset does not work.
• Typically, this involves inserting the key into the ignition and turning it to the "ON" position without starting the engine. Then, press a series of buttons on the key fob.
• After reprogramming, test the keyless entry system again.

Check for Faulty Key Fob

• If you have a spare key fob, test it on the car to determine if the issue is with the original fob.
• If the spare fob works, the original may be faulty. Consider getting it inspected or replaced by a dealer.

Consult a Professional

• If none of these steps resolve the issue, it may be a more serious problem with the car's receiver or antenna. It is advisable to take the vehicle to an authorized Mercedes-Benz dealer or a qualified automotive locksmith.
• The professionals have the appropriate diagnostic tools to identify and fix more complex issues.

Following these steps should help you address the most common causes of a malfunctioning keyless entry system on your 2017 Mercedes-Benz C-Class.